1. Field of the Invention
The invention relates to alkyl alkylaryl adipate plasticizers for polyvinyl butyral and to the production and use of plasticized polyvinyl butyral resin, particularly as an interlayer for liminated safety glass.
2. Description of the Prior Art
Plasticized polyvinyl butyral is widely used in the manufacture of transparent sheets and particularly as an interlayer in laminated safety glass such as glass for transportation vehicles, eyeglasses, shields, screens and in various architectural applications such as show windows, etc. While the art teaches a relatively very large numbers of plasticizers as suitable for plasticizing polyvinyl butyral, only a few plasticizers are known when the plasticized polyvinyl butyral is to be subsequently used as an interlayer in safety glass. One of the reasons for this is that polyvinyl butyral is a resin whose polymerized molecules exhibit wide variations in the total number of butyral and residual hydroxyl acetate groups that are present in the molecular chain, and in addition, the molecular masses of the resins vary greatly. The suitability of a given plasticizer is significantly dependent upon the foregoing features of the resin and particularly upon the number of residual hydroxyl group.
Therefore, among the known and typical plasticizers many are incompatible or at best only partially compatible with polyvinyl butyral resin--the most typical manifestation of incompatibility is the exudation of plasticizer from the plasticized polyvinyl butyral resin.
Furthermore, and for various reasons, many plasticizers which while compatible with polyvinyl butyral in that exudation does not occur, nevertheless do not provide the properties required when the resulting plasticized product is to be used in high performance laminated safety glass. Specifically, the polyvinyl butyral laminated glasses must possess not only good optical qualities but also the properties of good impact resistance--even at extreme temperatures--and a resistance to delamination or loss of adhesion between the sheets of glass. Conventional plasticizers yield a plasticized polyvinyl butyral resin which, when used for lamination, yields a laminated glass deficient in one or more of these properties.
Known plasticizers for polyvinyl butyral are the diesters of dicarboxylic acids, such as certain adipates, and especially those in which the diacid contains 4 to 14 carbon atoms. The alcohol fraction of the ester is made up of alkyl or alkoxyalkyl radicals containing fewer than 12 carbon atoms. U.S. Pat. Nos. 2,124,315 and 3,884,865, for example, mention methyl, ethyl, propyl, butyl, pentyl, hexyl and octyl adipates. However, these adipates have only partial compatibility with polyvinyl butyral, and even this partial compatibility decreases as the hydroxyl content of the polyvinyl butyral resin increases, or as the molecular mass of the adipate plasticizer increases. Also, those plasticizers which possess a vapor pressure greater than 10 mm Hg at 175.degree. C. tend to produce bubbles, delamination or other defects in the resulting laminated sheet.
On the other hand, the art teaches that plasticizers of the type consisting of glycol esters of carboxylic acids, or dialkyl or alkoxyalky esters of dicarboxylic acids, provide good flexibility at low temperatures, but such is not the case with plasticizers such as phthalates containing polarizable groups such as benzene rings. Among the publications dealing with such plasticizers, there may be mentioned in particular Kunstoffe, vol. 60, pages 301 to 308 (1970), which deals more particularly with plasticizers for polyvinyl chloride; U.S. Pat. No. 2,290,193, dealing with plasticizers for polyvinyl acetal; and Internation Polymer Science and Technology, vol. 2, pages 90 to 106, which relates to plasticizers for rubber.